Objective The improvement of super absorbent polymer (SAP) with various mass fractions on bulk density, porosity, capillary water migration law, and water retention of coal gangue matrix in mines has been investigated, and the water migration law of coal gangue matrix under the optimal usage of SAP has been elucidated. It provides a basis for improving the water condition of coal gangue matrix in soilless mining area, which exploits coal gangue solid waste resource utilization in mines, and provides suitable conditions for vegetation growth.

Method The effects of SAP with mass fractions of 0%, 0.1%, 0.2%, 0.3% and 0.4% on the water migration law of coal gangue matrix were simulated by indoor soil column test.

Result (1) There were physical property improvements of coal gangue matrix by containing SAP: the bulk density decreased with higher SAP usage amount, while the total porosity and capillary porosity showed an opposite trend. (2) SAP can significantly inhibit the rising height of coal gangue capillary water, and the mass fraction was in the range of 0%−0.4%. With the increase in SAP dosage, the inhibition effect was gradually enhanced. However, when the mass fraction of SAP was 0.4%, the inhibition degree of coal gangue capillary water was not significantly different from that of 0.3%. (3) It was obtained by (least significant difference, LSD) fitting analysis that the rising height of coal gangue capillary water increased as a power function with the time change (P < 0.01), and the rising rate decreased as a logarithmic function with the time change. (4) The coal gangue matrix water content with different SAP mass fractions was dropping continuously within 72 h in indoor ventilation conditions. The moisture content of pure coal gangue matrix (SAP mass fraction 0%) decreased the most with the moisture loss rate of 15.96%. The SAP mass fraction of 0.3% and 0.4% showed smaller downward trends and the water loss rate was 3.90% and 3.52%, respectively, with insignificant difference. And there will be more prominent water retention performance of SAP with a longer operating time.

Conclusion SAP can improve the water condition of coal gangue matrix, and within a certain range, the higher the content, the more significant the improvement effect. Therefore, from the perspective of economy and effect, SAP with a mass fraction of 0.3% is preferred to be applied to the coal gangue matrix in the soilless mining area to provide suitable water and gas conditions for vegetation growth.